1. Field of the Invention
This invention relates to the recovery of petroleum from subterranean reservoirs and more specifically to the recovery of petroleum from attic reservoirs by an improved gas injection process.
2. Description of the Prior Art
Many, if not most, petroleum reservoirs can be characterized as inclined and/or curved roughly laminar solids composed of porous rock into which petroleum has migrated. The petroleum accumulates in the reservoir because of the presence of an effective permeability barrier to the further upward migration of the petroleum at the stratigraphic top of the reservoir. This permeability barrier may be caused by several physical conditions. The presence of an impermeable caprock, faulting, and stratigraphic pinch outs being the most common. Often, but not always, petroleum gas will be present in the uppermost portion of the reservoir. Below this gas, and separated by the gas-oil contact would be found the oil. Beneath the oil would be the connate water of the reservoir rocks, the interface between the two being marked by the oil-water contact.
As the petroleum reservoir is developed as an oil field, the oil and gas are drawn off and the oil-water contact in the reservoir rises. This process continues until the oil-water contact reaches the structurally highest producing well in the oil field. Normally, this will mark the economic limit to the further development of the oil field and its subsequent abandonment. Nevertheless, in many reservoirs there remains a substantial volume of oil in that portion of the reservoir above the uppermost well and below the permeability barrier which forms a seal at the top of the reservoir. This untapped portion is called the attic oil reservoir.
The normal technique to produce this attic oil is to inject gas into the reservoir. Injection of the gas causes a downward displacement of the oil-water contact corresponding to the volume of gas injected. The injected bubble of gas rises to the top of the attic reservoir and displaces a corresponding volume of oil downward toward the producing interval in the uppermost well. This in turn allows resumed production of oil from this well. If this well begins to produce too much water again, signifying that the oil-water contact has again risen to the level of producing interval in that well, the gas injection process can be repeated.
This gas injection method is not without problems, however. As the uppermost well is produced, the water level beneath the well, marked by the oil-water contact, commonly will not remain as a relatively flat surface, but will instead form a cone with the apex at the producing interval in that well. This effect is commonly referred to as "water coning". It is caused by the difference in relative mobility between the connate water and the oil. When the uppermost well is put back on production, a low pressure area is created in the reservoir at the point of the producing interval in that well. The initial fluid to be produced will be oil, of course, since the oil should now surround the producing interval in the well. However, as this oil is produced, the oil-water contact beneath the well begins to be distorted upward due to the higher relative mobility of the formation water until the formation water again breaks through into the producing well.
It is readily seen that substantially more oil could be produced if this phenomenon of water coning could be corrected. In point of fact, there is no process at present that effectively attacks the problem, and the commercial need thereof is substantial. This invention provides a solution to water coning and recovery of oil from attic oil reservoirs.